1. Field of the Invention
The present invention relates generally to a thermal type flow detecting apparatus, and more particularly, to an improvement of a thermal type flow detecting apparatus for detecting the flow of a liquid or a gas by utilizing thermoelectromotive forces.
2. Description of the Prior Art
Conventionally known thermal type flow detecting apparatus are roughly divided into two types. A first type thereof detects the flow by heating a detecting portion of, for example, a platinum resistor, arranging the detecting portion in a fluid and measuring the change in electric resistance or current consumption of the detecting portion which appears by the cooling action of the fluid. The second type thereof detects the flow by arranging a heater separately from the detecting portion and measuring the change in electrical characteristics of the detecting portion caused by heat transfer by the fluid which occurs between the heater and the detecting portion. The change in electrical characteristics utilized in the second type comprises the change in resistance value, the change in thermoelectromotive force, the change in pyroelectric current.
In the thermal type flow detecting apparatus of the first type, i.e., a self-heating type, a detecting portion itself of a platinum register is heated. Thus, the life of an element is short. In addition, in order to improve sensitivity, the detecting portion, i.e., a heat generating portion must be made small. However, in such a case, the mechanical strength is decreased. Thus, the flow detecting apparatus of the first type can not be employed in an environment in which mechanical impact is liable to be given or an environment in which a fluid flows at high speed without reducing a sensitivity. Furthermore, since a noble metal such as platinum is used, the cost thereof is high.
On the other hand, in the thermal type flow detecting apparatus of the second type, a resistive heating body is frequently employed as a heater. In addition, as a detecting portion, a thermistor and a metallic resistor are employed or a thermoelectric couple is employed. However, it is difficult to miniaturize the detecting portion in order to improve sensitivity and increase the response speed. More specifically, in the flow detecting apparatus employing the resistive heating body as the heater, the resistor can be miniaturized and made thinner by thin-film techniques or the like. However, in the flow detecting apparatus employing the thermistor or the metallic resistor as the detecting portion, the number of manufacturing processes is increased by miniaturization, and the structure of an element and a signal processing circuit becomes complicated if the resistance value is increased, so that the cost thereof is liable to be high.
Additionally, the thermal type flow detecting apparatus employing the thermoelectric couple as the detecting portion, electromotive forces of the metal thermoelectric couple are small, so that detecting sensitivity can not be so increased.